CIJ printers are widely used to place identification codes on products. Typically a CIJ printer includes a printer housing that contains a system for pressurising ink; a print head located at or close to a point which items to be coded pass; and a conduit containing fluid and electrical connections linking the printer housing and the printhead. In operation, ink is pressurised in the printer housing and then passed, via an ink feed line in the conduit, to the printhead. At the printhead the pressurised ink is passed through a nozzle to form an ink jet. A vibration or perturbation is applied to the ink jet causing the jet to form into into a stream of droplets, a process known as break-up.
The printer includes a charge electrode to charge selected droplets; and an electrostatic facility, typically a spaced pair of conductive plates held at different potentials to create an extra high tension (EHT) field there-between. Those droplets that are charged are deflected by the EHT field away from their original trajectory and onto a substrate. By controlling the amount of charge that is placed on droplets, the trajectories of those droplets can be controlled to form a printed image.
A continuous inkjet printer is so termed because the printer forms a continuous stream of droplets irrespective of whether or not any particular droplet is to be used to print.
The printer selects the drops to be used for printing by applying a charge to those drops, these drops then being deflected by the electrostatic facility to subsequently impact a substrate. Uncharged drops are not affected by the electrostatic facility and continue, on the same trajectory as they were jetted from the nozzle, into a catcher or gutter.
The unprinted drops collected in the gutter are returned from the printhead to the printer housing via a gutter line included in the conduit. Ink, together with entrained air, is generally returned to the printer housing under vacuum, the vacuum being generated by a pump in the gutter line.
During operation of a CIJ printer, it is common for ink to build up around the printhead area. By way of example, micro-satellites associated with the break-up can be attracted to the deflector plates, or to the gutter. Over time this build-up can reduce the air gap between the deflector plates, or between one plate and ground, leading to arcing which in turn causes break-down of the EHT field.
Typically CIJ printers have a sensing facility to detect this arcing, initiate removal of the voltage supply to the plates, and then shut-down the printer. This avoids print quality being adversely affected. However a problem can arise in that the detection of this expected arcing can be confused with other sources of electrostatic discharge. For example, an operator holding a static charge may discharge himself by touching a metallic part of the printer. Such confusion is undesirable as the machine may shut-down when it is not valid or necessary, leading to a loss of operational effectiveness.
It is an object of the invention to provide a method of and means for addressing the above problem; or at least to provide a novel and useful choice.